1. Technical Field
The present invention relates to an exhaust gas purifying catalyst and to an exhaust gas purifying device having the catalyst for vehicles, and in particular, relates to an exhaust gas purifying catalyst in which nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO) contained in exhaust gas exhausted during a low-temperature operation of an internal combustion engine of vehicles can be simultaneously and efficiently purified and reduced.
2. Background Art
It is well known that noble metal elements (Pt, Rh, Pd, or Ir) exhibit superior purifying effects on exhaust gas such as CO, HC, NO and NO2. It is desirable to use such metals in an exhaust gas purifying catalysts. Ordinarily, these noble metals are used mixed with or supported by Al2O3 which is a support having high specific surface area, together with an additive such as La, Ce and Nd. On the other hand, complex oxides such as perovskite which can be used in combination with many kinds of elements, have extremely various properties. Therefore, it is desirable to use the complex oxide in an exhaust gas purifying catalyst. Furthermore, it is known that properties of noble metals will change if the noble metal is supported on the complex oxide. In view of this, an exhaust gas purifying catalyst in which a noble metal is supported on a complex oxide can exhibit further appropriate properties for purifying exhaust gas.
Many kinds of such catalysts for purifying exhaust gas have been developed. For example, Japanese Unexamined Patent Application Publication No. Hei 06-100319 discloses a technique in which a part of noble metal elements are contained in a crystal of perovskite type structure. In this technique, it is disclosed that the noble metal is contained in the crystalline lattice of this structure, the particles of the noble metal are miniaturized and dispersed more, lattice defect contributing catalytic activity is appropriately generated, and therefore, the catalytic activation of the noble metal element can be improved. Furthermore, as a technique similar to the above-mentioned technique, Japanese Unexamined Patent Application Publication No. 2004-41866 discloses a technique in which Pd is solid-solved in a perovskite type complex oxide to improve heat resistance. There has been a problem, in that in the case in which the noble metal is Pd, PdO which is an active species for NO reduction reaction, is reduced to Pd having low activity. However, Japanese Unexamined Patent Application Publication No. 2003-175337 discloses a technique in which reduction of PdO is reduced by using a perovskite type complex oxide of A site defect type.
However, conventional exhaust gas purifying catalysts exhibit sufficient efficiency to purify CO, HC, NOx (NO, NO2 or the like) in exhaust gas during operation of high temperature (above 400° C.) such as during running of a vehicle, although it cannot exhibit sufficient efficiency during low temperature operation (below 400° C.) such as at start of engine operation or idling operation.
The reason why it cannot exhibit sufficient efficiency to purify exhaust gas during low temperature operation is as follows. In the conventional exhaust gas purifying catalyst, noble metals such as Pt, Rh, Pd or the like are used while being supported on Al2O3 having high specific surface area. Since Al2O3 has high specific surface area, the noble metals are supported in a highly-dispersed condition. However, since Al2O3 is a stable compound and there is no interaction between Al2O3 and the noble metal supported, activity of the noble metal itself cannot be improved. Therefore, sufficient efficiency may not be obtained during low temperature operation.
It is desirable that Pd exist in a PdO-state having high activity during operation of vehicles. However, there is also a problem in that even if Pd initially exists in a PdO-state supported on Al2O3, it is reduced to be Pd in a metallic state at high temperature (above 900° C.), the Pd clumps, reducing the active sites, and as a result, activity is greatly decreased. This is because the supporting material Al2O3 cannot reduce mobility of Pd and cannot stabilize oxidizing conditions of PdO on the surface of the supporting material since the surface of the supporting material Al2O3 is extremely stable.